Fix bugs in cow_multipart:parse_body/2 when a partial was found at the end

There was three bugs: lack of a scope argument in binary:match (I swear
I had it in a previous version, whoops!); a misinterpretation of
the returned position when the scope is used (so yeah it wouldn't
have worked either way); \r\n being incorrectly removed when
we got a partial, despite not knowing whether it was a boundary.

Added a series of different tests to ensure that it all works as intended.
Updated cowlib to 0.5.1

Fix multipart body reading. Thanks to Ali Sabil for the many tests
and reports.
Add .gitignore